1. Field of the Invention
The present invention relates to an optical clock signal extraction device to be used for an optical repeater of a long distance large capacity optical fiber communication system, and more particularly to a device for extracting high-speed optical clock signals which exceed the upper limit operation speed of an electronic device.
2. Description of Related Art
In an optical communication network, the transmission distance is constantly becoming longer with larger capacities. With the distance of transmission becoming longer, deterioration of the quality of optical signals becomes a problem, due to the optical loss in optical transmission lines, a drop in the S/N ratio by the user of optical amplifiers in multiple steps, and waveform distortion by group velocity dispersion, nonlinear optical effect, and their combined effect in optical fibers. The generation of waveform distortion in the frequency domain and the waveform distortion in the time domain become more conspicuous problems as transmission capacity increases.
Because of this, repeaters are installed in the middle of an optical transmission line with an interval of several tens to several hundred kilometers, and optical signals are regenerated, that is, the waveforms in the frequency domain and the waveforms in the time domain of the optical signals are restored to the original forms by the repeater. One major role of the repeater is clock signal extraction. Clock signal extraction is generating a pulse output (or sine wave output) signal corresponding to the bit rate from the optical signal formed by the optical pulses in which the waveform in the time domain is distorted, that is an optical signals of which quality deteriorated.
A clock signal is extracted either as an electric signal or an optical signal, depending on the case, and in the following description, a clock signal may be referred to as an electric clock signal and optical clock signal only when necessary to clarify which form the clock signal is extracted. Frequency corresponding to the bit rate of the optical signal indicates the frequency f when the bit rate of the optical signal is f. For example, if the bit rate of the optical signal is fGbits/s, the frequency corresponding to the bit rate of the optical signals is assumed to be fGHz. Hereafter the frequency corresponding to the bit rate of the optical signal may be called the “bit rate frequency”. In this example, the bit rate is denoted in Gbit/s and frequency is denoted in GHz, but in the following description, the unit is not limited to this. Therefore in the following description, unless necessary, the unit notation may be omitted.
One general method conventionally known as a clock signal extraction method is a method of extracting only the frequency components corresponding to the bit rate of an input optical signal by inputting an optical signal, of which quality deteriorated, into a photodiode performing photo-electric conversion and filtering the output electric signal from the photodiode by a band pass filter. Hereafter an optical signal, to be the target of extracting the clock signals including optical signals of which quality deteriorated is called an input optical signal.
An optical pulse string is generated by generating an electric clock signal using a photodiode and band pass filter, and operating an optical pulse laser device, such as a laser diode using the electric clock signal. This optical pulse string is a string of optical pulses lined up on a time axis at a cycle of repeat frequency which corresponds to the bit rate frequency of the input optical signal. In the following description, an optical signal refers to an optical signal generated as an RZ (Return to Zero) signal, which is a binary digital signal. This optical signal is a signal generated by performing optical modulation on a string of optical pulses which regularly line up at a predetermined periodic interval on a time axis. The expression “optical pulse string” is assumed to be all the optical pulses which regularly line up at a predetermined periodic interval on a time axis.
Generally the photoelectric conversion characteristic of a photodiode does not depend so much on polarization thereof, so even if a time-based fluctuation exists on the plane of polarization of the input optical signal, a clock signal can be stably extracted by using a photodiode.
On the other hand, as a technology to increase the transmission capacity of an optical communication network, a multiplexing transmission technology, such as optical time division multiplexing, is under research. A bit rate of the multiplexed signal is a bit rate per multiplexed channel, multiplied by the number of channels, so it is a very high bit rate. Hereafter the bit rate of a multiplexed signal may be referred to as the “transmission rate”, and the bit rate per channel as the “base rate”.
If the bit rate of the multiplexed signal exceeds 40 Gbit/s, it is difficult to extract a clock signal in the electronic device. This is because a photodiode which operates even at 40 Gbit/s or higher bit rate optical signals and an electric narrow band filter which operates even at 40 GHz or higher electric signals have not been developed.
In order to extract a clock signal from a high-speed optical signal, a method of extracting the optical clock signal directly without performing photoelectric conversion is used. Hereafter this method of extracting optical clock signals directly without performing photoelectric conversion may be called the “all-optical clock signal extraction method”.
As the all optical clock signal extraction method, a method of using a mode-locked laser has been reported (e.g. see T. Ono, T. Shimizu, Y. Yano and H. Yokoyama: “Optical clock extraction from 10-Gbit/s data pluses by using monolithic mode-locked laser diodes” OFC '95, Technical Digest, ThL4). Also a method of using a self-excited pulse generation laser, such as a self pulsation laser, has been reported (e.g. see M. Jinno and T. Matsumoto: “All-optical timing extraction using a 1.5 μm self pulsating multi-electrode DFB LD”, Electron. Lett., Vol. 24, No. 23, pp. 1426-1427, September 1988).
In any of these methods, the input optical signal is input to the mode-locked laser or self pulsation laser which generation optical pulses at a repeat frequency close to the bit rate of the input optical signal, and extracts optical clock signals by synchronizing the output optical pulses from the mode-locked laser of self pulsation laser with the bit rate of the input optical signal.
One advantage of these methods is that a high-speed clock signal can be extracted as an optical clock signal, which is impossible with an electronic device, as mentioned above. For example, a reported example is that the extraction of an optical clock signal from 160 Gbit/s optical signal succeeded (e.g. see S. Arahira, S. Sasaki, K. Tachibana and Y. Ogawa: “All-optical 160 Gb/s clock extraction with a mode-locked laser diode module”, IEEE Photon. Technol. Lett. Vol. 16, No. 6, pp. 1558-1560, June 2004).
However the above mentioned conventional all-optical clock signal extraction method has the following problem. That is, the operation for optical clock signal extraction depends on the polarization direction of the input optical signal. In order to extract the optical clock signal by a mode-locked laser and self pulsation laser, the polarization direction of the input optical signal and polarization direction of the oscillation light of the laser must be matched.
Therefore if the polarization direction of the input optical signal changes for any reason, the optical clock signal cannot be extracted stably. Generally, the input optical signal propagates through a single mode optical fiber where maintaining the polarization direction is not guaranteed, and is input to an optical repeater, for example. Therefore a method which can extract an optical clock signal stably even if the polarization direction changes must be considered based on the assumption that the polarization direction of an input optical signal to be input to the optical repeater changes along with time.
With the foregoing in view, it is an object of the present invention to provide an optical clock signal extraction device which can extract an optical clock signal without depending on the polarization direction of the input optical signal.